When an engine goes through a cold start condition a secondary air flow fan can be used to inject air into the engine's exhaust system. The reason the air is injected into the exhaust system is so that oxygen is present in the exhaust system and causes excess hydrocarbons to be combusted. This also helps the catalytic converter to perform efficiently or achieve optimal temperature in a shorter amount of time.
An impeller fan can be used to create the air movement in the secondary air flow system. One phenomena that can occur with secondary air flow systems is what is referred to as “dead head” condition. A dead head condition is when the air flow or output channel from the impeller becomes blocked. In other words, due to impeller design the pump will reach relatively high pressures at dead head and prevent the downstream valve from closing.
Furthermore, as the pressure increases the electrical current drawn by the motor increases. This is an undesirable condition because it is a drag on the vehicle electrical system. Therefore, it is desirable to develop an impeller that would reduce the pressure at the dead head condition, and thus reduce the amount of current drawn by the impeller.